The Importance of Preheating Before Welding and Post-Weld Heat Treatment

Preheating before welding and heat treatment after welding are very important to ensure welding quality. The welding of important components, the welding of alloy steel and the welding of thick parts all require preheating before welding. The main functions of preheating before welding are as follows:

(1) Preheating can slow down the cooling rate after welding, facilitate the escape of diffuse hydrogen in the weld metal, and avoid hydrogen-induced cracks. At the same time, it also reduces the hardening degree of the weld and heat-affected zone, and improves the crack resistance of the welded joint.

(2) Preheating can reduce welding stress. Uniform local preheating or overall preheating can reduce the temperature difference (also called temperature gradient) between the workpieces to be welded in the welding area. In this way, on the one hand, the welding stress is reduced, and on the other hand, the welding strain rate is reduced, which is beneficial to avoiding welding cracks.

(3) Preheating can reduce the constraints of the welded structure, especially the constraints of corner joints. As the preheating temperature increases, the crack occurrence rate decreases.

The selection of preheating temperature and interlayer temperature is not only related to the chemical composition of the steel and welding rod, but also related to the rigidity of the welding structure, welding method, ambient temperature, etc. These factors should be determined after comprehensive consideration. In addition, the uniformity of the preheating temperature in the thickness direction of the steel plate and the uniformity of the weld area also have an important impact on reducing welding stress. The width of local preheating should be determined according to the constraints of the workpiece to be welded. Generally, it should be 3 times the wall thickness around the weld, and should not be less than 150~200mm. If preheating is uneven, not only will the welding stress not be reduced, but it will be increased.

Post-weld heat treatment has three purposes: eliminating hydrogen, eliminating welding stress, and improving the weld structure and overall performance.

Post-weld hydrogen elimination treatment refers to the low-temperature heat treatment performed after welding is completed before the weld has cooled below 100°C. General specifications are heating to 200~350℃ and keeping warm for 2-6 hours. The main function of post-weld hydrogen removal treatment is to accelerate the escape of hydrogen from the weld and heat-affected zone, which is extremely effective in preventing welding cracks when welding low-alloy steel.

During the welding process, due to uneven heating and cooling, as well as the constraints of the component itself or external constraints, welding stress will always be generated in the component after the welding work is completed. The presence of welding stress in components will reduce the actual load-bearing capacity of the welded joint area and produce plastic deformation. In severe cases, components may be damaged.

Stress-relieving heat treatment is to reduce the yield strength of welded workpieces at high temperatures to achieve the purpose of relaxing welding stress. There are two commonly used methods: one is overall high-temperature tempering, which is to put the entire weldment into a heating furnace, slowly heat it to a certain temperature, then keep it warm for a period of time, and finally cool it in the furnace. in the air or in the furnace. This method can eliminate 80%-90% of welding stress. Another method is local high-temperature tempering, that is, only heating the weld and its surrounding area, and then slowly cooling it to reduce the peak value of the welding stress and make the stress distribution relatively flat, thereby partially eliminating the welding stress.

After welding some alloy steel materials, the welded joint will have a hardened structure, which will deteriorate the mechanical properties of the material. In addition, this hardened structure may cause joint damage under the action of welding stress and hydrogen. After heat treatment, the metallographic structure of the joint is improved, the plasticity and toughness of the welded joint are improved, and the comprehensive mechanical properties of the welded joint are improved.